This invention relates to gas lasers, especially to diffusion-cooled CO2 lasers with improved pulsing performance.
Diffusion-cooled CO2 gas lasers have a number of useful industrial and medical applications. Traditionally, most commercial diffusion-cooled CO2 gas lasers utilize a radio-frequency (RF) excitation discharge that is transverse to the laser""s longitudinal axis. RF excitation allows for a very compact, low cost, all-metal laser tube designs, which are well known for their long life-time and reliability. Less popular, though still used mainly in medical CO2 lasers, direct current (DC) discharge excitation, that is parallel to the laser""s longitudinal axis, allows for less expensive (than RF) DC power source. It is often desirable to have a laser capable of emitting a continuous wave (CW) low power output and also capable of emitting a pulsed high power output. Unfortunately, RF excitation becomes very expensive to implement if high peak powers are required. On the other hand, the longitudinal DC discharge cannot be easily modulated which limits the pulsing rate of laser with DC excitation. However, a combination of RF excitation with DC excitation could overcome the limitations mentioned above if such excitation utilizes the low power RF and high power DC excitation circuits.
A prior art excitation method utilizing transverse RF discharge and longitudinal DC discharge is described by V. I. Myshenkov, et. al. in xe2x80x9cStability of a composite discharge sustained by static and RF electric fields. II. Mechanism for stabilizing effect of an RF field on the positive column of a DC dischargexe2x80x9d, Soviet Journal of Quantum Electronics, 1982, vol. 8, No. 4, pp. 397-400. This prior art method describes the diffusion-cooled CO2 laser, which includes a glass discharge tube filled with the laser gas. A pair of long RF electrodes is located outside of the glass tube parallel to its axis so that the transverse RF discharge inside the tube is created when RF voltage is applied to the RF electrodes. A pair of short DC electrodes is located inside the glass tube on the opposite ends of the tube so that the longitudinal DC discharge inside the tube is created simultaneously with the RF discharge when DC voltage is applied to the DC electrodes. A disadvantage of this method is in the use of the glass discharge tube, which is fragile and expensive to manufacture.
Another prior art laser with combined transverse RF and transverse DC excitation is described in U.S. Pat. No. 5,097,472 (the ""472 patent) issued to Peter P. Chenausky. This laser design utilizes two elongated metal electrodes to which both the RF and DC voltage is applied. The excitation circuitry includes the RF and DC power sources as well as the elements to electrically isolate one power source from the other. The pulsed RF voltage is applied to the electrodes and a uniform transverse RF discharge lights up in the interelectrode gap. Once the RF discharge is established, it commutes the energy from the DC power source through the electrodes and through the interelectrode gap filled with the RF discharge plasma.
A disadvantage of the technique described in the ""472 patent is a very high cost of the RF power supply required to pre-ionize the gas at elevated gas pressure. Indeed, a 28 kW RF pulse is described for commuting a high power DC discharge through a 1 cm interelectrode space between 13.5 cm long electrodes in a CO2 laser operated at 225 Torr. An average laser power output from the described electrode configuration can not exceed approximately 5-10 W for diffusion-cooled laser gas (Vitruk et. al., xe2x80x9cSimilarity and Scaling in Diffusion-Cooled RF-Excited Carbon Dioxide Lasersxe2x80x9d, IEEE J. Quantum Electronics, QE-30, 1994, pp. 1623-1634), which translates to approximately $2,000 price for commercially available CO2 lasers. At the same time the price for the 28 kW RF power source is in excess of approximately U.S. $30,000 for a vacuum tube based design, which makes this approach prohibitively expensive.
Another disadvantage of the method described in ""472 patent is a very low efficiency of the transverse DC excitation if a low pressure gas discharge is used. Indeed, a voltage across a 1 cm long DC gas discharge at 30 Torr is described in ""472 patent as 400 Volts. It is well known in the art of electrical gas glow discharges that approximately 250-300 Volts is distributed across the cathode fall region, which does not contribute constructively to the creation of the laser gain in the DC discharge. Therefore, no more than approximately 30-40% of the DC power dissipated in the transverse DC discharge is actually deposited in the plasma column, in which the laser gain is created.
Another prior art laser with combined transverse RF and transverse DC excitation is described in the U.S. Pat. No. 5,596,593 (the ""593 patent) issued to Katherine D. Crothall et. al. This prior art laser includes two pairs of elongated electrodes, which form a four walls surrounding a square bore of a CO2 gas laser. First electrode pair is used to create a transverse RF discharge, while the second pair of electrodes is utilized to sustain a transverse DC discharge. Similarly to ""472 patent, a disadvantage of the method described in ""593 patent is a low efficiency of the transverse DC gas discharge excitation due to significant voltage drop across the cathode fall region of the DC discharge.
It is an object of present invention to simplify and improve the combined RF and DC excitation method used in high peak power, diffusion-cooled gas lasers. Present invention allows for a very simple, light-weight, low-cost, long life-time, all-metal design of the high power, efficient pulsed gas laser for either industrial or medical applications.
The present invention overcomes the above discussed and other disadvantages of the prior art by providing a novel and improved technique for achieving a combined RF-DC gas discharge in the optical cavity of the gas laser. A method for producing a beam of laser energy with the all-metal gas laser according to present invention consists of the following steps. First, a plurality of elongated metal electrodes is provided, at least two of the electrodes having a plurality of dielectrically coated surface areas and a plurality of metal surface areas located on a discharge surfaces of the electrodes. Second, the spacing between the metal surface areas is provided to be substantially greater than the spacing between the discharge electrodes. Third, an RF voltage is applied between at least two electrodes to establish a transverse RF gas discharge between the discharge surfaces of the two electrodes to which the RF voltage is applied. And finally, a DC voltage is applied between at least two electrodes, thereby producing a longitudinal DC gas discharge through said transverse RF gas discharge with the DC gas discharge current flowing between the metal surface areas of the two electrodes to which the DC voltage is applied.
The combination of the fast pre-ionization with the transverse RF discharge and the high power excitation with the highly-efficient longitudinal DC discharge allows for a high-speed modulation of the active medium of the high-power gas laser. At the same time, the all-metal electrode system allows for a very compact, reliable and low cost design of the gas laser.